Run program when/instead of writing to FIFO?

I have a program that writes data every second to a FIFO. Now I want to alter some of this data and write it to another FIFO. What would be the best approach? Can I somehow pipe this directly to my program (I have no control over/source code of the original program)? Or would I have to create a program that runs in the background and constantly polls the FIFO for new input?